1. Field of the Invention
The invention pertains to the field of hydraulic tensioners for imparting an appropriate tension. More particularly, the invention pertains to a hydraulic tensioner having a ratchet mechanism to prevent a plunger's retraction at the time of a decreased hydraulic pressure.
2. Description of Related Art
A hydraulic tensioner generally includes a housing, a hollow plunger that is slidably fitted into a bore formed in the housing that is biased in a protruding direction by a spring, and a fluid chamber defined by the bore of the housing and the plunger. During operation, a force of a chain or belt imparted on a distal end of the plunger balances a resilient force of the spring and the hydraulic force present in the chamber.
A force from a chain is imparted on the distal end of a plunger when an inadequate amount of hydraulic pressure is present in the chamber, such as engine start, the plunger is easily forced to retract into the housing, thereby causing noise or oscillation. In order to prevent such a retraction of a plunger, various kinds of hydraulic tensioners with ratchet mechanisms have been proposed, such as shown in Japanese Patent Application Publication Nos. 2000-136859 and 2001-304360.
The ratchet mechanism shown in the Japanese Patent Application Publication No. 2000-136859 includes a rack that is supported translatably in a longitudinal hole formed in a housing, a ratchet that is fitted slidably in a lateral cavity formed in the housing that has a plurality of teeth adapted to engage the rack, and a spring that is fitted in the cavity that biases the ratchet in such a direction that each tooth of the ratchet engages with the rack.
The ratchet mechanism shown in the Japanese Patent Application Publication No. 2001-304360 includes a rack groove formed on a circumference of a piston, a claw member that has a plurality of teeth adapted to engage the rack groove that is slidably fitted in a lateral cavity, which extends in a direction intersecting an axial center line of the piston in the housing, and a spring that is fitted in the lateral cavity that biases the claw member in such a direction that each tooth of the claw member engages with the rack teeth.
When the force of a chain is applied to the distal end of the plunger of the above prior art ratchet mechanisms the plunger moves in a backward direction, engaging the rack with the teeth of the ratchet or the claw member, preventing the plunger's retraction. However, the mechanisms that achieve this result are complicated in structure.
Therefore, there is a need in the art for a hydraulic tensioner with a ratchet mechanism that is simple in structure and prevents retraction of the plunger when an inadequate amount of hydraulic pressure is present in the chamber.